Whilst cooking. I just can't do it really. I think 9/10 times it detracts from the recipe in question. Say, if you're making spaghetti bolognaise, and it seems like a good idea to throw in some peppers or mushrooms - it just never is. You end up with this generic meat and vegetable pasta sauce. Whereas if you stick to recipe properly and hone it, how much garlic and how to cook it, what kind of tomatoes give the most flavour etc... it can be just sublime.
I'm like that though. I find a recipe and make it as perfect as I can, and then I'm unwilling to change it. Like stir-fry. Noodles, garlic, spring onion, sweet basil, oyster mushroom, salt, ground pepper, light soy, shoyu sauce, and pan seared tofu (cooked separately and marinated in ginger and dark soy). Perfect! But the second a pepper or a bit of mange tout or sweetcorn or a beansprout goes near it, it just gets half as good in an instant.
Do you have 'classics' you don't like to mess with? Or do you chuck in whatever.