Publish and/or Perish
Murder: Jessica Fletcher (Lansbury) is a retired teacher who has written a series of novels (seemingly unconnected) under the name J.B. Fletcher, which seem to have left her well-known and well-off. She's good at solving cases because they often bear a striking resemblance to the plot of one of her many, many books. Later on in the series, she once again becomes a teacher of writing and criminology; man, we would not want to be students in her class. The murder rate must be ridiculous.
Castle: Richard Castle (Fillion) is a bestselling author whose novels about detective Derek Storm have made him a celebrity in the publishing world -- and the serial killer world; the series begins with him helping the NYPD figure out who's staging copycat murder scenes straight out of his books. But even aside from the ones he's already written about, he seems to be able to solve crimes because he knows what would make the best novel. ...Well, the best hour-long crime drama, anyway.
DNA Match? They're both successful authors, both have large bodies of work, and both see elements of their stories in crime scenes. Maybe they should have called this show Son of Fletcher.

A Police Matter
Murder: Fletcher seems to encounter crime everywhere she goes, be it her home of Cabot Cove, Maine, or across the United States or even in jolly old England. While her hometown sheriff -- first Amos Tupper (Tom Bosley), then Mort Metzger (Ron Masak) -- always appreciates her help, most other law enforcement agencies tend to frown on an old lady's involvement until she starts being right. Although they should probably just arrest her, for being a magnet for murder.
Castle: When the series begins, Castle has just killed off his main character, and he decides to go on a permanent ride-along with the lead detective in the copycat case, Kate Beckett (Stana Katic), to study her for a new character he's creating. The chief seems to think it's a good idea, although Beckett seems to find him annoying, despite being a fan of his work. Hopefully that annoyance will fade after he comes up with the breakthrough that solves the case for her for the 15th time.
DNA Match? Jessica seems to have a more easy-going relationship with law enforcement, but then, she also isn't as obnoxious and condescending.

It's a Family Affair
Murder: While she's widowed and lives alone, Fletcher has the biggest family of any TV character we've ever seen, and that includes the cast of Brothers and Sisters. Every other episode sends her off to visit a relative, however distant, from her niece Courteney Cox to her wacky, identical twin cousin in England. She's like an elderly Patty Duke!
Castle: Castle doesn't have to travel far to visit his extended family -- they all live and work right alongside him. His teenage daughter Alexis (Molly Quinn) and his mother Martha (Susan Sullivan), a former Broadway actress, share his fancy apartment with him, and although we don't know much about his first wife, his second (ex-)wife is also his hard-nosed publisher. Could Castle be a widower, as well? And is he somebody's wacky identical twin? (Oh please oh please oh please...)
DNA Match? Okay, so Castle's family isn't as big as Jessica's, but they might both be widowers, which will give them something to talk about at the crime novelists' convention.

New York Groove Grave
Murder: Fletcher may be best known as the most famous resident of Cabot Cove, Maine -- giving it one of the highest crime rates in the country -- but she later moved to New York full time, where there were even more crimes to solve. Of course, not living in a small town, it makes it a little more suspicious that she knows so many of the victims.
Castle: Castle is pretty much the king of New York City. He knows everybody, and has probably dated half of the women in town under 30. When Detective Beckett says "NYPD," he simply says "NY." ...Of course, the show is filmed in Los Angeles, but MSW's Cabot Cove scenes were filmed in California, as well.
DNA Match? Jessica isn't exactly New York born and bred, but she and Castle both seem to have their fingers in a lot of the Big Apple's crime.

With Friends Like These...
Murder: When she's not visiting her massive family, Jessica is visiting friends around Cabot Cove and across the country, most of whom will get accused of murder at some point and need Jessica to bail them out. Aside from the many policemen she meets who are fans of her books, Jessica can count multi-millionaires, homeless people and Magnum, P.I. among her friends. That's right -- Magnum P.I..
Castle: Castle plays cards with other famous authors, including James Patterson and Stephen J. Cannell, and he has a massive fan base. The more important fans get the honor of being his friends... whom he can call and ask favors of. That list includes the mayor of New York City and at least one NYC circuit judge, which is helpful if you need to speed along a DNA test or a search warrant.
DNA Match? Both have friends (and fans) in high places, although Jessica knowing Magnum P.I. beats Castle knowing Michael Bloomberg, hands down.

Heart Attack!
Murder: Unfortunately (or is that fortunately?), sexagenarians and sex don't often go together. Despite smooching a co-star in the first episode of the series, that was the last time Jessica made goo-goo eyes at anyone, leaving the romance for the kids and respecting the memory of her dead husband.
Castle: Castle seems to be more antagonistic with his ex-wife/boss than anything else, and he seems to enjoy signing the cleavage of his many nubile fans. Also, he and Detective Beckett seem to have a fairly obvious flirtation going on, which should attract Bones viewers after Booth and Brennan hook up this season and everyone loses interest. Hell, even his mother is man-hungry, possessing acute "graydar" for single, older men.
DNA Match? Jessica doesn't have the libido that Castle (or his mother) does, and never gets cozy with anyone that we see. Well, maybe Tom Bosley, but can you blame her? Father Dowling's mysteries, indeed!

Read our interview with Nathan Fillion, then read our recap of the first episode of Castle, and discuss it all in our Castle forums!