Listen first
Your next line is in what they just said.
Because you both had an idea, and neither of you heard the other one.
The most common beginner habit is composing your next line while your partner is still talking. The audience can see it, and the scene stops being about anything.
Once listening is real, the rest follows quickly: relationships establish themselves, the unusual thing becomes obvious, and heightening has somewhere to go.
Playing is the only way to practice it. Every session here is scenes on your feet with immediate notes, which is why reading about improv never seems to help anyone.
Your next line is in what they just said.
Scenes work when nobody is competing.
A performance practice, coached honestly.
Twenty minutes, free, on where your play gets stuck.