Wow. I look at headshots used by actors, and often just cringe. Bad lighting, bad cropping, bad choices in hair / makeup / jewelry / clothing / expression, bad white balance -- bad, bad, bad. And what is the result of using a bad headshot? Well, that answer is simple: the casting director will pass. Especially in today's digital world. Your headshot will flash across a busy director's computer screen (or worse, smartphone screen), and in that 0.6 seconds you either get their attention, or not.
Headshots are an actor's primary tool to get new business. They are not glamour shots. An overly edited beautiful picture is as bad as a horrible shot, because it makes you look like you want to be a model, not an actor. The CD (casting director) will probably laugh, and then pass.
As with any tool, you need to spend the time to get it right, to select the appropriate tool for the job and then make sure it gets used correctly. You should have several tools in your work bag, because no tool is perfect for all situations. When you want that steamy romantic lead spot, you won't want to give them your villainous look, and vice versa. How many looks do you have? That's how many good headshots you should have.
The tool needs to work for the casting director, too. This means there can be no surprises that could embarrass the CD when you show up on set, or even just waste the CD's time when you're called in for an audition. If the CD selects your headshot, then you need to show up looking like that picture. Headshots need to be updated regularly, at least annually, and certainly after any major change (new hairdo, tattoo, scar). Children grow so fast, they need to update at least every 6 months.
Finally, buy solid tools you can trust. Don't go to the equivalent of Harbor Freight by having your friend-with-a-camera shoot your headshot, and don't got to just any photographer. Choose a photographer with acting, film and/or stage experience, who knows how to create your tools. Your photographer needs to know the acceptable and desired standards in your industry, because there are differences in commercial (film and television) vs. theatrical headshots, there are different standards Los Angeles vs. New York City, there are even different trends year by year as to what CD's are looking for.
Think of your headshot as a tool, and use it to your best advantage.