You need to take this very seriously, and retain legal counsel to defend you. You may have a good defense if you can establish that your IP address was not used to download the movie----but you should not assume that you are off the hook merely because your IP address does not exactly match the IP address on the letter. My guess is that you have already been sued by Malibu Media LLC in a Federal Court for copyright infringement as one of several "John Doe" defendants, and that based on that suit and with court permission, the attorneys for Malibu have issued a subpoena to Comcast to attempt to identify the persons associated with IP addresses that were used to download illegal materials. If you retain counsel, and if you can prove that your IP address was incorrectly named in the letter from Comcast or the law suit, you might be able to obtain an order blocking Comcast from revealing your name and personal information (although it is very hard to obtain such orders). Once Malibu has your personal information, it will demand that you pay an amount to settle the case. You need legal counsel to defend yourself in this situation---it would be a mistake to simply ignore the letter. It may be that you have valid defenses to the claim, and if so, your legal counsel may be able to convince Malibu to drop the case. But there is a good chance that your correct IP address was, in fact, associated with an illegal download, and if so, you will need to consider whether you have any other defenses, or whether it is in your best interest to settle the case. This is one of those situations in life in which you need a lawyer---and there really is no substitute for obtaining one.
Your question implies that you pay extra to Comcast for an unchanging (static) IP address. The usual situation is to have an IP address that's assigned individually each time you log in (i.e. dynamically). It's more likely that Comcast has correctly identified your account based on their records.
Expect Comcast to provide your contact information to the lawyer for the copyright owner, unless you make an appropriate challenge to the request (potentially expensive) or reach a settlement with the copyright owner.
I have handled such matters for Internet users so accused and am familiar with the issues that arise. Typically the copyright owner sues the individuals identified in such a trolling operation. Frequently the copyright owner claims that the downloaded material was a pirated copy of a pornographic video, which may embarrass some individuals to have themselves publicly so identified. If one elects to engage an attorney to deal with the copyright owner's lawyer, one may often preserve anonymity in such an extortionate situation, if the matter is resolved promptly.
This posting is intended for general education and isn't "legal advice." It doesn't create or evidence an attorney-client relationship. You are encouraged to engage an attorney in the pertinent jurisdiction for confidential legal advice on matters of any importance.
There are quite a number of attorneys who specialize in downloading cases. Any one of them would be far more efficient, and likely far more effective, in handling your downloading case than would some local attorney you track down. See, for example:
You need an attorney who is already authorized to practice in the Court where the lawsuit was filed. Yes, a lawsuit has already been filed and you're one of many John Does [which is the only way the copyright owner could have sent your ISP the subpoena]. Hiring an attorney who's already authorized to practice in the Court will save you much time and money.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.