How does Twain begin to build the climax of Jim's escape plan by using the element of suspense?

Twain begin to build the climax of Jim's escape plan by using the element of suspense by not giving too much details to the readers about Jim, so it makes people curious. Once again, Tom is making the decisions, while Huck merely plays along, and Jim simply accepts. Interestingly, Tom is still the same boy he was when the reader last saw him in the earliest chapters of the novel. However, Huck has developed into a more mature, morally sound individual. Huck always thought Tom's make believe adventures were not worth the time or effort Tom put into them. But, here, he believes they are truly setting Jim free, and releasing him from the bonds of slavery. For Huck, this is one of the most serious and risky actions he has ever undertaken, but for Tom, it is all just a game.

Here is his plan from Chapter 39 by the way!

Tom and Huck first plant a letter reading, "Beware. Trouble is brewing. Keep a sharp lookout." The next night the boys tack up a letter containing a skull and crossbones, which they follow with a picture of a coffin.

Tom plans a final coup by drafting a longer letter. Pretending to be a member of a gang of robbers who are planning to steal Jim from the family, he warns them that the gang will be coming late at night from the north to get Jim. The family is terribly frightened and does not know what to do.