Is it the same as "speaking on behalf of" ? Or does "speaking for someone else" mean trying to give an opinion about something that someone else is experiencing without you yourself going through that experience ? If so, what is a single word or a phrase that can be used to express that ?
Let's say a bunch of friends including John and myself are discussing about going on a picnic. I know that John doesn't like picnicking. So I say, in the presence of John, "John doesn't like to go on picnics". And John says (annoyedly) "speak for yourself". In this context, which one is meaningful; "speaking for John" or "speaking on behalf of John" ?