My cousin is getting married this Saturday at my former Catholic parish. Though I left religion, I still study it objectively. While I will be at the Mass, I won't be participating in it. Rather, I will be observing closely and taking mental notes.
Honestly, not even the most conservative Catholics cared enough about the actual Mass. Thankfully, all focused on my newly wedded couple rather than doctrine and religion, a far cry from the last wedding i was at.
Many of the people at a Catholic Mass will be friends of other faiths, or none and not expected to partake of the sacraments. I wouldn't worry about it.
I don't have a problem in participating in the rituals, although I've never taken part in a Christian mass and suppose I might draw the line there. But in terms of being present, I usually enjoy the singing and the atmosphere in Churches, though I'm less enamoured of the sermons.
I found out i am catholic in 2004. With the new pope i like it more but i am a happy non believer
You know the routine, you will be probably more seriously aware then those who go every week. Just smile and it will all be over before you know it. They cannot read your mind but your pretty much into what they are thinking. I would advise to study it subjectively as their is much to learn from the Lutherans, Methodists and others. I was in Israel for six weeks and went to Mosques, Temples, various churches for other reasons. Nothing can compare to the Cathedrals of Europe which are Museums in themselves which took a century or more to complete and we don't know who the artists were. Spain is another out of mind experience and you get tour guides who are not religious but historically as accurate as they know how to be. Enjoy your cousins wedding and you don't have to tell them anything.