Many actors are a bit casual at memorizing the actual lines in the script. This is not only disrespectful to the typically absent playwright; it also usually hurts the play. If you’re one of these actors, there is little I can say to convince you to handle the matter otherwise, and I’m not going to try.
But when it comes to another actor’s cue, be sure to give the cue line verbatim. Tiny little things can throw an actor off, and if you start messing with your word choice when delivering a cue, you can, at best, take an actor out of the moment, and it will take him a half page to fully integrate himself with the play again. At worst, you will cause him to forget his lines. He is memorizing not only his own lines, but his cues, and he isn’t memorizing what you say in rehearsal, but what is in the script. (Probably in the vain hope that you will someday stick your nose in it and figure out that you’re saying the line wrong.)
So be courteous, and deliver the cue lines as written. Every time.