The 'Step In' (a la Tony Almeida on 24 series 4) is that point when a man/woman who you really like/love is talking to you or just standing close and you have no choice but to step in for the kiss.
Works best when interrupting someone in mid sentence.
"How did your date with Andy go last night?"
"It was fantastic, we went for a walk on the moonlit beach, it was so romantic, he was staring into my eyes when I was talking when suddenly he Stepped In and kissed me.'
"How was your date with Sarah?"
"Cool, we went to the beach."
"Cool. You Step In?"