Two characters talking · Phosphene docs

Eight renders of “two aliens at dinner” in one day taught the panel a lot the hard way. Before rendering a ninth, this is what Lightricks’ own guides, the LTX-2 model card, the community workflows and the open issues say about dialogue, multiple characters and continuity — and the recipe that follows from it.

1. Lip-sync with several faces in frame is an open problem, not a prompt bug

Lightricks’ ComfyUI issue #395 (“How to control who is speaking… when there are multiple characters in the image?”) describes exactly what the owner heard: “Character A or B starts lip syncing randomly to parts of the dialogue… very often they both appear to talk the same words simultaneously.” It is open, with no maintainer answer.
The community’s working answers are all the same shape: one speaking face per generation (render one character at a time and composite), masks with external audio, or pre-recorded dialogue. “Insisting” in the prompt who talks (“the woman in the red dress is talking, the man is silently listening”) helps sometimes and fails often — which is what our v2/v3 measured.
The audio guide’s own advice: one speaker per line, short lines, the speaker and delivery written before the quote (“she says in a calm, seasoned voice: ’…’”), ~9 s of speech at a natural pace, the word “lip sync” in the prompt, a well-lit front-facing face, and “let the face arrive first” — a line that starts on frame one gives the sync nothing to lock onto.

So: a conversation is written so that only the speaker’s face is on screen while a line plays. The panel’s One Shot staging rule (a beat names only who should be in the picture) is the same idea; it is just the wrong container.

2. The right container is a multi-shot generation, not a one shot

LTX-2.5’s headline feature is native multi-shot: several connected shots with explicit cuts inside ONE prompt and ONE generation, with the model keeping “character identity, environment, lighting and voice” across the cuts because it reasons about the whole sequence at once. Lightricks’ 2.5 prompt guide:

“Use a single continuous take when you want unbroken camera motion, intimate performance, or dialogue that must stay lip-synced in one framing. … When a scene involves dialogue, multiple beats, or precise timing, write it in a screenplay style.”
“Prefer 2–4 shots in one generation.” At every cut: name the edit in prose (“a hard cut to…”), re-establish the new shot (scale, angle, who is in frame, light), re-identify returning characters with the same descriptors, never a pronoun, and say what the sound does (“the room tone continues across the cut”, “the dialogue drops; only wind remains”).
Lock what must persist in words: “Preserve her face, red coat, blue-magenta lighting… across every shot.”

A two-person conversation is therefore shot / reverse shot inside one generation: a medium shot of A saying its line, a hard cut to a medium shot of B at the same table in the same light answering, the room tone continuing. One face per shot, so nothing lip-syncs in unison; one generation, so it is one table, one pair, one voice each. The “two different tables, the character changes in the middle” of the long-table take was the cost of stitching three separate generations of a scene the model was never allowed to see whole.

3. Continuity across generations, when a scene must outlast one

Community extension workflows (RuneXX, Kijai, the IAMCCS extension nodes) feed the last ~3 seconds (73 frames) of the previous clip as context — not one frame — colour-match the new segment to the old, blend the audio, keep segments 10–20 s, and expect drift after ~60 s. “Incorrect slices can result in weird seam timing, sudden resets, or continuity breaks.”
The panel’s One Shot hands off ONE frame. That is why every join stalls and restarts (measured this morning) and why a scene drifts across parts. The vendored RetakePipeline.extend_from_video conditions on a source clip; fed a 73-frame tail, it is the community method. The earlier “windows chain” that went to mush used a 121-frame tail on Q4 — a different experiment.
The vendored fork also carries Prompt Relay (--segment TEXT [LEN]): each segment’s words are gated to a slice of the timeline inside one generation. It cannot stop a visible mouth from moving, but it can keep speaker B’s line from bleeding into speaker A’s seconds. The panel does not expose it yet.

4. The recipe for the aliens (and any two-hander)

One generation per exchange, 10 s (241 frames) at High 720p on this Mac, written as a screenplay-style multi-shot prompt:

Night, a candle-lit dining room. A medium shot of a tall thin alien in a deep
violet robe, pale grey-green skin, seated at a small round table, warm
candlelight on its face, alone in frame. It looks up and says slowly, in a deep
resonant voice: "The humans split the atom before they learned to share bread."
A hard cut to a medium shot, same table, same candlelight, of a short round
alien in a plain grey robe, darker olive skin, wide head, alone in frame; the
room tone continues across the cut. It answers quickly, in a thin high voice:
"Every species we ever watched said that about its neighbours." Preserve both
aliens' faces, robes and the candlelit room across every shot. Lip sync.

Rules baked into it: one speaker per shot and per line; the speaker described before the quote, every time, with the same words; the cut named; the sound across the cut stated; the “preserve” sentence; “lip sync”; the face arrives before the line. A 30 s conversation is three such generations, joined on cuts (a cut hides a seam; a one shot cannot), each re-describing both aliens with the same descriptors so the model re-creates the same pair. If a scene must be one unbroken take longer than 10 s, extend with a 73-frame tail as context, not a frame (§3) — that path is unbuilt in the panel today.

4b. Two faults measured on a 90 s monologue take, and their fixes

Voice over a closed mouth at the end of parts and on the first frame: 10–14-word lines in 5 s beats. Speech runs ~2.5 words/s, so the audio overran the beat and was cut at the part boundary. Fix: ≤7 words per beat, the silence after the line written, a silent look before the first line (the panel’s own duration rule; §4 already assumed it).
“Cuts blended with an overexposure effect”: an image-to-video part drifts in exposure inside itself (mean luma 137 → 178 across ten seconds) and the next part re-levels one frame after its anchored first frame (178 → 157), so a boundary colour match sees nothing to fix. Fix: scripts/join_smooth.py --hold-exposure pins every frame’s mean luma to the take’s first second (smoothed over 12 frames) — flat at 159 across the same 30 s that swung 126–175 — plus the seam colour fade and a 120 ms audio crossfade. Valid only when the light is constant by contract, which a one shot’s light lock makes true; a scene that is meant to darken must not use it.

4c. A talking character across parts: hand off on a talking frame, gate the sync

A continuation part anchored on a closed, silent mouth will not lip-sync the next line (measured: mouth-opening-vs-voice r −0.15 / +0.20; the community’s extension note says the same). Anchored on a frame from mid-word it syncs (+0.47). One Shot’s take_handoff=speech trims each part where its line ends (take_speech_end, 0.4 s pad) and anchors the next part there; each part’s first beat opens with a silent look so the sync has a moment to lock.
Even a first part is a roll: identical settings gave +0.41 and −0.27 on two lines. So every spoken part is measured (take_lipsync_score, the same measure as scripts/lipsync_check.py) and retaken once under 0.20 with a fresh seed; the better clip is kept.
One real line (≤18 words) per 10 s part, never a line per 5 s beat; no cut/edit/shot/scene words in the dialogue; a trained face renders on Q8 Pro.

5. Proof (rendered 2026-09-07, 10 s, High 720p, seed 4242)

The §4 prompt, submitted as one ordinary text-to-video job (no take, no handoff): 0–5 s the tall alien alone at the candle-lit table saying its line; a clean hard cut at 5 s; 5–10 s the short alien alone at the same table answering, brightly lit, expression readable. One face per shot, one room, one pair — the first render of the day where nothing could speak in unison and nothing changed tables. mlx_outputs/ltx_multishot_dialogue_aliens_10s_high720p.mp4.

Sources

Lightricks, LTX-2.5 prompt guide — https://ltx.io/blog/ltx-2-5-prompt-guide
Lightricks, Prompting guide for LTX-2 — https://ltx.io/blog/prompting-guide-for-ltx-2
Lightricks/LTX-2 README and the LTX-2 model card (structure, “frame count 8k+1”)
Lightricks/ComfyUI-LTXVideo issue #395 (multi-character lip-sync, open) — https://github.com/Lightricks/ComfyUI-LTXVideo/issues/395
RuneXX/LTX-2.3-Workflows discussions #116 (dual character) and #131 (multi-extend: 3 s of context frames, colour match, 20 s groups, ~60 s before drift)
Kijai/LTXV2_comfy discussion #36 (extend any video: 73-frame minimum overlap)
fal.ai, How to use LTX-2.5 (multi-shot: name the edit, re-establish, re-identify, state the sound; ≤3 shots; let the face arrive before the line)
ltx23.org, LTX 2.5 release guide (native multi-shot, “Preserve … across every shot”)
ltx.io/blog How to add AI voiceovers & dialogue (one speaker per line, short lines)