Be a calming presence
The groom is likely to be feeling a lot of stress and anxiety on the day of the wedding. It’s up to the groomsmen to help calm him down and keep him focused on what’s important. This may include helping him relax with some light banter or keeping things moving so that he doesn’t have time to worry about anything.
Be a source of support
The groom is going to need all the support he can get on his wedding day, and it’s up to the groomsmen to provide it. This includes being there for him when things go wrong (which they inevitably will) as well as lending an ear when he just wants to vent. It’s important to be a good listener and offer helpful advice when needed, but ultimately the decision of what to do should be left up to the groom.