This is not strictly a Mark Twain posting, but it is a close relation.
Hal (Holbrook, not Bush) will be guest starring on the episode of NBC's 
"ER" airing April 24 at 10 p.m. As many of you know, Hal came through a 
terrible illness that caused the cancellation of "Mark Twain Tonight!" 
performances for the last four months of 2007. At 83, he is back strong, 
doing Twain dates when time and schedule permits. Pretty amazing. 
Attached are the top paragraphs from the NBC press release issued today. 
Sounds as if he's playing a character close to him in spirit:

Burbank – March 27, 2008 – Multi-Emmy Award-winning actor and recent Academy Award nominee Hal Holbrook makes a special guest appearance on NBC's "ER" (10-11 p.m. ET) as Walter Perkins, a terminal cancer patient who impacts the life of Dr. Kovac (Goran Visnjic).  The episode, "Truth Will Out," will air Thursday, April 24 (10-11 p.m. ET).

During a visit to a hospice ward, Dr. Kovac crosses paths with Walter, who despite his terminal illness, remains upbeat, offbeat and funny -- while continuing to outlive every other patient that comes through the doors of the facility.  His presence impacts Luka and affects a major decision in his life.

Executive Producer David Zabel said, "The show has had an amazing array of incredible actors who have walked through the doors over the years and Hal Holbrook certainly continues that tradition.  He is perfect for this role and we couldn't be more pleased to have him."

A Tony Award recipient and five-time Emmy winner, Holbrook is one of the great craftsmen of stage and screen. He is best known for his performance as Mark Twain, a role for which he won numerous accolades for "Mark Twain Tonight!"  He has appeared in more than 50 television movies and mini-series including "Pueblo," "Our Town," "Sandburg's Lincoln" and "Outer Limits."  He also starred in the landmark television movie "That Certain Summer."  Holbrook has guest-starred on such television series as NBC's "The West Wing," "NCIS," and "The Sopranos." 
 
On the big screen, his career began with "The Group" in 1966 and since then, he has appeared in over 40 films.  These include "Magnum Force," "Midway," "All the President's Men," "Wall Street," "The Firm" and "Men of Honor."  Earlier this year, Holbrook became the oldest male performer ever nominated for an Academy Award for his supporting role in "Into the Wild."