This really depends on the time frame you have in mind, since this practice changed quite a bit over the centuries. The earliest records of this specific type of suicide were in the final years of the Heian Period (794–1185) by Minamoto no Tametomo (1170) and Minamoto no Yorimasa (1180). It became more frequent over the following centuries and during the Muromachi period (1336–1573) it was already a very common practice. At this point, any samurai would be willing to do it, in case his lord demanded. It was the main form of execution of the time for the nobility (the samurai). Instead of being condemned to be hang or beheaded, a samurai would've simply been commanded by his lord to take his own life.

During the Edo period (1603–1868) it became highly ritualized, involving assistants, a specific type of clothing, a last meal, a last cup of sake and the writing of a death poem. It also became a form of judicial punishment, so some people were sentenced to commit seppuku, although this was abolished in 1873. With the Meiji restoration (1868) and the abolishment of the samurai class, the practice fell into disuse, although some people have still done it since then, like the famous japanese author and poet Yukio Mishima in 1970 and the japanese judoka Isao Inokuma in 2001.

See this thread and this thread and this thread

I haven't read the popular novel. But no a samurai wouldn't be willing to commit seppuku at the slightest opportunity. While there were non-punishment reasons for seppuku, by far the most common was as a death sentence, or if the samurai knew he was going to be handed one anyway. The samurai's dead. Seppuku just allow him to separate himself from common criminals or traitors who got beheaded/crucified, and maybe as a final gamble as part of sentencing/negotiation to save the lives of his other clan members.