I don't think genetics have the biggest component in attaining that look. I think it does depend a lot on where you're starting from, whch genetics have some control over, but is more likely a product of lifestyle. In Fight Club, the physique that Brad Pitt has is fairly light and lean. If you want to look like that, don't try to be a bodybuilder. If you start out as a shrimp, it would be easier to attain that sort of physique than if you started out overweight, but I think for most people it would be an attainable goal, because most people don't naturally have huge muscles.
I think it would be ideal to start out skinny when going for this look, then begin a hypertrophy program (if you are overweight, I'd focus on losing weight healthily until you're down around a BMI of 25 or less first, then focus on building muscle). Don't approach it the way most guys go about bulking (which is eat whatever you can fit in your mouth). Instead, eat healthy, eat what you need for maintenance, and then add only a couple hundred kcal on top. The goal is to gain muscle without gaining fat. The more fat you gain, the harder it will be to guage where you're at progression-wise, because this is the sort of look that most likely won't even be noticeable if your body fat % is double-digit. Depending on where you're starting from, you might only need to gain 5kg, or you might need to gain 20kg. I'd aim to gain only 1kg/month. If you're doing quality resistance training, then that 1kg will be muscle.
Don't overemphasise your abs. It doesn't take a lot to build up your rectus abdominis, so spending 20 min each session trying to build it up is probably a waste of time. One of the main reasons a lot of guys never see their 6 (or better yet, 8) pack is because that's all they worry about. Your big compound lifts are what's going to build most of your muscle and burn excess fat. Your slight caloric surplus and balanced nutrtion is what's going to build you up and get you into shape. Any more than 5min spent on abs in a session is most likely not giving them any extra benefit, whilst it's taking away from the benefits you can be achieving around the rest of your body.
Once you achieve the weight you're looking for (which I suggest would be around a BMI of 25 with <15% body fat), you should then focus on getting rid of any excess fat while maintaining muscle. This means keep doing full-body resistance, do things like HIIT, keep up the nutritional quality of your food, and reduce kcal to slightly below maintenance. Aim to lose excess weight as slowly as you gained muscle mass. Once your body fat is down to single digits, you will appear more cut, and your muscles will actually appear bigger, even though you're actually smaller. To use myself as an example of the effectiveness of this optical illusion, I currently weigh 62kg and have a BF% of about 15%. I'm a lightweight, and everyone knows it. I used to weigh about 55kg back in high school, but had only about 7% BF, with a little bit of muscle on me. It made me look ripped, and even though I was underweight, people at school and at work all thought I was huge.
Once you've got most of your muscle on at a normal BMI and have a single-digit BF%, you should have attained the type of physique that Brad Pitt had in Fight Club. If, after all that your abs still need more work, THIS is the time to worry about it, because your BF% is low enough that your abs will be visible, and they won't need a lot of work to touch up (if you somehow managed to get to this stage and had no visible abs, I can almost guarantee that 1 month of more specific training would make them completely visible. But ultimately around this stage, you're looking at maintenance of body composition. I'd be moving onto some sports-specific training, so that you can toughen up to be about as hardened as you look.
