One should not take his 3 steps backward after davening shmoneh esrei if there is a person directly behind him still davening . I believe this is true no matter how far the person behind him is, but I'm certain that it's true if he's less than 4 amot away. Since he is not able to take his steps back to complete shmoneh esrei, I would imagine that he is not able to reply to all (or perhaps any) parts of davening that would normally be responded to such as barukh hu uvarukh shemo and amein to the berakhot of chazarat hasha"tz, kedusha, amein yehei shemei rabba, etc. What kinds of responses are permitted in this state?
Assume that the person has said yihyu leratzon... but has not yet taken three steps back and said oseh shalom.