Do the changes in the following sentences change the meaning at all?
He's my friend
I'm his friend
Or
He's my co-worker
I'm his co-worker
Or
he's my brother
I'm his brother
Besides emphasis - is there any change in meaning in these sentences?
Can they be interchangeable? (Assuming friends are not just one sided!)
Yes, it depends on whether the relationships are symmetric. Brotherhood is not symmetric, because if you're a girl and he's your brother, then you could say
he's my brother
but switching it and saying
I'm his brother
is clearly wrong, you'd say
I'm his sister.
In other words, "He's my brother" doesn't imply anything about your gender, but "I'm his brother" clearly does, so the two phrases have different meaning.
The "co-worker" relationship is symmetric. "Friendship" is debatable. You might consider someone your friend but they may not consider you their friend, sadly.
I agree with what Amit wrote, but would add besides, that in each case the two pairs are not interchangeable because of the point of view of each sentence of each of the pairs. In one case, it's from your point of view (first person singular) in one of the sentences, and from the other person's point of view (third person singular) in the other. If the relationships were described in the third person plural, e.g. "we're friends", "we're brothers" (or if one is male and one female, "we're siblings", or "we're co-workers", these would be equivalent to both parties.
First, this is not about "interchanging roles in sentences". In the sentence
there is only one noun with a role: that noun is Bill and that role is Subject.
The other noun, friend, is not referential, like Bill -- i.e, it doesn't refer to an individual.
Instead, it's the Predicate of the sentence. It's called a Predicate Noun.
What you're talking about, and what you'd probly figure out if you started looking at more and different kinds of sentences, are Reciprocal Verbs.
That is, (be) s.o's friend is a reciprocal predicate -- if Bill is my friend, then I am Bill's friend.
Not only that, but it is also true that Bill and I are friends (note the plural predicate noun).
The same can be said of (be) s.o's <kinship-term>(s), and (be) s.o's co-worker(s)/neighbor(s).
Among many others.
