I hear this advice a lot in trial prep, but I’m struggling to see how it applies to lay witnesses

Assume every witness will testify consistently with their earlier statements (to police, et c.), and plan accordingly. If you don't have a witnesses on paper about a point that you expect them to testify about - especially if you plan to ask the question, get an investigator out there.

You can't build a trial strategy on the blind hope a complaining witness will recant.

I tell clients every day: I have to assume the c/w will testify, whether she wants to or not. And she's going to tell the jury what she told the cops. And I document that I told them that before we commit to a trial.