How can I distinguish a freeze frame from the moving parts of the video?
Add a sound effect: the effect fades in, is audible at full volume shortly before it abruptly ends exactly when the freeze frame appears. The sound effect should be distinguishable from background sounds and overall be crisp and short (like 0.5-2 seconds). That way, the audience is given a clear indicator that the freeze frame is intentional. An alternative might be a sudden tempo/rhythm change in music - but I wouldn't stop the music.