Take your role seriously but don't overdo it By definition, you are a GUEST Wedding Photographer, therefore you are not the primary wedding photographer.
It is the primary wedding photographer's main obligation to document the whole shebang.
He is the lead pack, so he gets the first bite.
Rather than competing with him, try to view yourself as his partner.
Complement, not rival, his work with your own.
When he's out taking pictures of the bride and groom's first kiss, go out and take pictures of the teary-eyed relatives watching them.
Try to move out of his responsibility circle and look for other subjects he may not be able to capture.
This way, more quality work is done and less redundancy, not to mention elbow wars is avoided.
Circulate Yes, socializing is the best way to being able to see minute details that the photographer may have missed.
The main photographer will probably be busy focusing on the bride and groom's every move.
Guests, even if they are part of entourage or are close friends, will have to take a close second.
So, move around.
Mingle with people.
Take candid photos.
Try different angles.
And vary subjects from the guests at the buffet table to the bridesmaids dancing.
The couple will surely appreciate what went on during their wedding after they've come out of their spell and take the time to browse those photos.
Whereas the wedding was intimate and romantic, the wedding reception will be a time to let loose and bring out loud guffaws.
Capture those moments.
Try to photograph each table even if the bride and groom aren't around.
Be Discreet Again, try to respect the bride and groom's day but not taking the spotlight away from them or distracting them from more important matters.
Wear suitable clothes for the event.
You'd want something that can blend in with the couple's theme and dress code.
Research beforehand what this might be and balance this with comfort and looseness to allow you more room to take difficult angles.
Do not use flash during the wedding ceremony itself.
Even if the church has low lighting, try to compensate this with high ISO.
Lastly, try not to be too obtrusive of the couple's most intimate of moments.
Don't tag along when they discreetly move out of the party place and unto their honeymoon.
They would want to be alone during these moments.
A photograph of it might be more daunting than rewarding in the long run.
Good luck to you!