Ryan Reynolds is naturally quite lean - he started at 11% bodyfat (stretched over a fairly decent sized frame). According to Men's Health, he supposedly dropped down to 3%. However, who knows how accurate that measurement was and how it was determined.
If you watch Blade Trinity closely, you'll see that he isn't all that big. In fact, with his clothes on he looks like a scarecrow. When he does have his shirt off towards the end, he is nicely bronzed, spritzed and in low light with generous shadowing working in his favor. If you had the same team setting you up for a scene like that, you'd probably look pretty good too
I will say that his diet was reportedly tight and he didn't work out much in the past. Men's health says he gained twenty pounds, but then again he started on creatine too. Here's the link you want to check it out:
http://www.menshealth.com/cda/article/0,2823,s1-4-0-0-2190,00.html
Another guy who went through a similar transformation was Thomas Jane who starred in The Punisher. Similar tight diet, rigorous excercise and a big fat paycheck for an incentive. Like Reynolds, Jane got pretty ripped but didn't appear like anything special with his clothes on. He looked buff in shots wearing a tight T-shirt though.
All things considered, I have seen guys cut bodyfat and put on muscle at the same time. The more out of shape someone is, the more dramatic the results naturally. Putting on additional muscle reportedly burns an extra 50 calories/pound/day. Add 10 pounds of muscle, and suddenly you're burning an extra 500 calories a day. I haven't tracked down the reference for this yet, but from what I've seen over the course of 20 years of on-and-off-again lifting, I believe it. Haven't seen anyone get bodybuilder big of course, but I've seen alot of Reynolds and Jane-like transformations (sans roids).
I'm experiencing that right now. In the last two and a half months, I've dropped my bodyfat from 20% to 15% and maintained my weight at 174 pounds. It has been a very noticeable metamorphasis and I've gotten some nice comments at the gym. HST has been key, along with a
very tight diet. Additionally, I run twice a week, swim twice a week and mountain bike when I can (training for an Xterra triathlon). I also think my age has something to do with it. When I was younger I couldn't gain muscle without bulking like mad. Among other things, I drank a gallon of milk a day! However, now that I'm 36 my metabolism now drags to the point where 2100 calories a day is working for me.
My progress is slowing, but I think I can eventually get to 10% bodyfat and push to 180'ish. Patience is key.