cinderella (and toi)– Perhaps you should discuss your question with the person who gave the seminar. It may be that they meant you should not “bond” with the person you are doing Kiruv with over things that are assur, or attempt to ingratiate yourself with them by showing that you are also “with it.” In your example, where the girl asks point blank “did you see that movie?”, were they recommending you lie and say “no, frum Jews don’t watch movies” even if you did? That sounds strange to me. I agree you shouldn’t respond “wow, it was great, I thought it was hysterical when . . . .”. Sometimes you can explain why we don’t do certain things without getting personal, and avoid the issue, but if her response is “you mean you never watch movies?”, I still think a response like “I try very hard not to, although this is an area that I am still working on” is better in the long run then a lie, which if eventually discovered will certainly lead her to view you as hypocritical.
Toi — My experience in Kiruv has generally involved adults, not kids at risk, so perhaps for kids it is different. I have never come across someone who got the message that I “can’t deal” with Halacha or it is “not worth it” when they found out for example that I find myself occasionally doing serious aveirahs such as speaking Loshon Harah. The usual reaction was the opposite — that even though I am FFB, I don’t just go on auto pilot and assume I’m good enough, but I am working on myself in the areas that are difficult because I truly believe in this and value it.
Additionally they get the message that we really believe what we preach. I know of someone who started becoming interested in Yiddishkeit after coming across a copy of the Chofetz Chaim’s sefer Ahavas Chessed, which is a Halacha sefer. The person found the book inspiring because he said “Everyone talks about being nice — but you guys actually have tons of technical rules about it. Not just ‘give charity’, but how much, who comes first, when not to give, etc. This shows your really MEAN it! It’s not just a nice philosophy!” Sharing your struggles can show this is a real way of life.
Again, I am not saying to make this person your personal confessor! But if I topic comes up, I don’t believe that telling lies in the long run will serve your best interests, and I don’t believe that is necessary to avoid learning any topic or Halacha that you have trouble with. I can even see a positive result if when learning a sefer on Tznius you explain in the beginning that tznius is required by Halacha and something you also struggle with, and you hope by learning together you can both become better in your avodas Hashem.
I’m not sure where your comments on them liking you more come in. I have never experienced anyone liking me more or less as a result of my struggles in avodas Hashem. How much others like you is usually a result of your middos and interpersonal skills, not whether you keep all of Halacha perfectly or have some areas where you struggle.