Coworker has the habit of interrupting me when I am talking to customers at the counter?

I work in a small office that does some walk-in sales. My coworker sits close to the counter and will listen to my conversations with customers and then interrupt me. I am not talking about a stray suggestion here and there. He will deliberately get up out of his seat and start addressing the customer as if you are not talking to them. This has happened to myself and another coworker on numerous occasions. I feel that it makes me look unknowledgable about our products and services.

In the past, I've just waited until he had completely taken over the conversation and then returned to my desk to complete other work. My opinion was that if he wanted to do the extra work, he could have it. We don't work on comission. I know for a fact that I make more money than he does. I also know he wants a raise, so I'm wondering if this is his way of making himself look better and "deserving of it".

Any suggestions to diffuse or correct this situation?

Thanks for the suggestions. I have spoken to management about other problems that are similiar in nature that I've had with this coworker.

I 'm not quite sure why he does this, as it normally makes quite a bit more work for him. He tends to be a bit of a know-it-all, so when he does this, he can let the customers know all about what he knows on a subject, whether they want the information or not.

My other coworker rebuffed him quite well one day. He interrupted him with a customer and then asked for help. My coworker replied, "It looks like you've got it covered" and sat down!

I will be trying as hard as possible not to have to talk to management again, but I felt very undermined when it occurs.

I would not say anything in front of a customer or another employee... that tends to make a person defensive...I would tell him that you do not appreciate him butting into your conversations when you are dealing with a client..it is very unprofessional of him and if he would like to handle a customer then say so before you are already talking to them..good luck

I would turn around and ask something like, "Don't you have any of your own work to do?" Or say something like, "I am already handling this, if I need your help, I will ask for it. You can go sit back down now."

Take him aside and tell him you know he wants a raise but trying to make you look inept is not the way to get it. If he doesn't stop take it to the boss, because you are right it demeans your professionalism.

I can suggest that you find an appropriate time within working hours to ask this person if they would mind speaking with you in private. Employ a management technique that you may be familiar with already... it's simplistic though it sounds that it may suit him and your needs perfectly.
First compliment him... TWICE. eg. I admire that you are forthright and that you are aware of the need to furnish our customers with all the relevant details concerning our products and services. However, I find that your interupting me whilst with a client and 'taking over' belittling and derogatory. This not only reflects poorly on your ablilities as an access point for customer service in front of a customer by acting in this dismissive manner, this also reflects poorly on our Company. I appreciate your eagerness and your skills as a colleague but I would respectfully request that you hold your counsel until I should ask for it.
Bit wordy, I know, but you can work around that.

Tip 1. Practise what you want to say, WELL, prior to confronting your work mate.
Tip 2. Should this fail to make a satisfactory impression then inform management (If, as you mention, you have another co worker with the same complaint then ask him/her if they are willing to endorse your quibble)
Tip 3. Purchase a cattle prod and fry the little git in front of a customer... basic dog psychology; If he piddles on the carpet then one short, sharp jolt should modify his behaiour significantly, or your money back!
Finally, no job is perfect... you're going to meet the odd jerk whereever you go... chin up and remember, rise above... you're better than that,.
Good Luck my little taco.
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Perhaps talk to a manager/higher up person. His actions may not affect you, but if they give a bad impression of the business interaction on the customer, your work could lose business, which is something the manager should address.

You don't want to do anything to embarrass yourself or your co-worker in front of the customer, or anything that would make the customer uncomfortable.

Just tell him! I think that the next time it happens, and after the customer leaves, go to him and tell him, directly: "Look, I know you're trying to be helpful when I'm working with a customer, but I need you to stop interrupting me when I'm working with a customer. It's disrespectful and it undermines our company with the customer, as if you don't trust me to give them the right information, and it makes us look bad."

I did not read the other answers, but I really think you ought to just "blast him good" after it happens the next time (and the customer has left). Tell him in no uncertain terms that you will not tolerate another interruption when you have a customer, and if it happens again that you will embarrass him totally, and then report it to the manager. Do all this in private, or with your other co-worker. I think he'll get the point.