I like to knit in the evening while I'm watching TV. I'm really bad at just sitting still so this is about the only way I can sit through a whole TV show. So here are my rules. Your mileage may vary.
1. Knit during the commercials. This is the best time to get some serious knitting done without missing the good parts of the show.
2. Stop knitting if the suspense gets to much for you. I don't generally watch scary shows but I do like mystery shows and if the actions get to suspenseful you could tense up. You could end up with an hourglass shape in the middle of your knitting because the tension went from nice and easy to oh my gods tight.
3. Don't knit from charts. The constant looking back and forth will make you either screw up your knitting or miss your show. This is not the time to do complicated intarsia.
4. Do knit patterns with lots of repeats. Simple lace, simple cables, plain textured knitting are all good choices for TV watching.
5. Wait to count rows or stitches until the commercials. You'll be less distracted.
6. Don't watch really funny comedies. One rerun of Coupling was enough to make me laugh so hard 3 stitches had fallen off the end of the needles before I even noticed.
7. TV time is great for boring knitting. TV is what gets me through 96 rounds of stockinette on that really big felted bag.
8. You can't eat and knit and watch TV. Generally you can't eat and knit any way but this is especially true while trying to watch TV. But then this is not a disadvantage if you're dieting.
9. Don't let your knitting take up the entire couch. If you live alone you can pretty much ignore this rule but I have to share my couch with 4 cats a husband and 3 kids. Husband gets grouchy when there is no where for him to sit because of all the yarn, patterns, notions, needles, et all. He also gets upset when he sits on double points, go figure. The cats just walk all over everything and lay on my knitting any way.
10. Be prepared. Have your knitting, patterns, notions, a drink and everything handy. So you don't have to get up. You can also ignore this rule if you have children you can send to fetch you the things you need. First you have to teach them what a cable needle is though, and you run the risk of them playing with your measuring tape.
All rules can be ignored if you're in the same room as the TV but aren't actually watching it. Husband likes to have me in the same room when he's watching movies. He calls this spending time together. He is mistaken. He is watching TV. I am ignoring the TV and doing something constructive. We are merely in the same room. Oh well, it makes him feel better.