Muscles need to be in proper balance to work together optimally. Our posture and movement assessments will tell us which appear to be too active or short and which appear to be underactive or extended. We need to stretch the short muscles and strengthen or activate those that are long or extended. Stretching and strengthening go hand in hand to achieve flexibility; the genera rule is that when a muscle is tight the opposite muscle is extended.
Here's an easy way to think about this: those muscles that you sit on all day long, your glutes, are probably asleep; the opposite muscles, hip flexors, are flexed all day long and remain short. The hip flexors can get so tight that they inhibit the glute. So stretch the hip flexors, strengthen the glutes, try not to overdo it on exercises that flex your hips... and you are on your way.
Do the same type of assessment and programming for other area of your body.