Does insanity still work if you don't stick to the diet plan but don't over eat either?

You will still lose weight regardless of your diet plan as long as you burn more calories than you take in. However, you will experience slower results. However, everybody has different dietary needs and lifestyles so a "one-fits-all" plan is not necessarily the best thing. To make sure you really aren't "over-eating" I would look for a BMR calculator to calculate the approximate amount of calories you should be taking in for your height and weight. Then I would keep a food journal to track your food intake.

Some additonal "diet" tips are to keep your diet full of whole, unprocessed foods. You will find that natural foods such as fruits, vegetables, and whole grains will not only keep you full, but will provide more consistent energy. Eat small meals throughout the day to keep your metabolism going, and drink plenty of water.