BBQ sauce that has simmered too long on the stove becomes thicker than desired. The flavors become more pronounced as well. If it's a spicy sauce, the heat level increases the thicker the sauce becomes. You can use bottled BBQ sauce as a marinade to get flavor into the meat rather than just sitting on the surface. Instead of diluting thickened sauce with water, which will make it weaker, dilute it with ingredients found within the sauce.
Tomato-Based Sauce
Read the ingredients on the label or the recipe in your cookbook. Instead of water you want to determine which ingredient will retain the flavors in the sauce. Quite a few commercial sauces are based on tomato sauce with spices and sugar added. Ketchup has those ingredients.
Add equal parts ketchup with beef broth if you're barbecuing beef or chicken broth if you're barbecuing chicken or pork.
Add 1/4 cup of the ketchup mixture to 1 cup of the BBQ sauce at a time, tasting after each addition until the consistency and flavor is to your liking. If the sauce is not tomato based, thin it with chicken or beef stock.
Vinegar Based
Remove 2 tbsp. of the BBQ sauce and set it aside.
Add 1 tbsp. of apple cider vinegar to 1 cup of the BBQ sauce until you've reached the desired consistency.
Taste the reserved sauce. Compare it to the thinned sauce. If the thinned sauce isn't as spicy as the reserved sauce, add a few drops of hot sauce. If it's not sweet enough add 1 tsp. of honey, maple syrup or brown sugar. Keep adding the hot sauce and/or honey until both sauces taste about the same.