Give the other characters a voice
Your lines stay silent (or use your own recordings); the other characters need audio. Two ways:
AI voices
Each other-character line has a voice selector — generate speech per line or for the whole import at once. Set your preferred default under Settings → Default AI Voice (with previews) and every new selector starts there. Regenerating a line replaces its audio.
A real scene partner
From the scene editor's collaboration panel, invite someone by email and choose which characters they should record. They get a secure link, see only their assigned lines highlighted, and record straight into the scene — their takes are attributed and immediately playable in your rehearsals. Collaborators can also practice the shared scene themselves. Full details — permissions, ending a collaboration, what happens to recordings — in Collaborate on a scene.
Mixing is fine: give one character to a friend and let AI cover the rest. Your own lines only get audio if you record them (or import via Record All) — that recording is what plays back as confirmation in With Gaps.