As a former Protestant myself here is my suggestion:
watch out for pride.
If you don’t know the answer, say “I don’t know.” The very best thing about the CC is that it has the truth. Lean on that truth and don’t tell fibs.
Don’t whitewash the truth. Sometimes they won’t like what you have to say but just grin and bear it. If you whitewash it, then you’re agreeing with their premise that the CC would be better if only it looked more like a protestant church.
Don’t be a star. They might convert because of you if you are a bona fide saint but otherwise you’re just there as a go between for the H.S.
Don’t fight their church. They’ll return the favor and they’ll win because they have the media and less history on their side. Becides God HAS been good to them – trying to convince them otherwise is a lie. A better option is to show them where they are in step with the CC. The “under God” clause in the pledge of allegence works good here in America and also serves as a wonderful foil against anti-democracy charges.
Don’t be afraid to get dirty When conversation turns to the naughty bits of the Church, don’t chicken out. Stay in there and argue that the church should be judged fairly. Clearly present the Catholic sense of right and wrong. Don’t let them believe that Catholic morality is: Catholics can do no wrong. (That’s the default Protestant belief of Catholic morality by the way.)
When the Spirit moves them, don’t interupt! You don’t have to have the last word on everything. Similarly when they outgrow you, let them go. Help them find the books or people they need to continue their journey.
Confused? Ask them to paraphrase what verses mean to them. Some verses have aquired extra significance that a Catholic or Protestant may be unaware of. Being “born again,” or founding the church on Peter for example.
Fighting? Try to say “Here is what the church teaches” and “Here is how we read this verse” instead of insisting that it’s the right way. By doing so you can turn the conversation from confrontation to exploration.
Arguing endlessly about a greek root word? Don’t get bogged down. Conceed the point for arguements sake and play devil’s advocate with parables.
Lastly, don’t give up.
Oh, one more thing – judgements like: “you’re too sinful to acknowledge that the Catholic church is right!” or “You’re stupid and you’re gonna STAY stupid until the Holy Spirit sets you streight” count as giving up.