
Foreign Veterans come in to school Japanese troops in the gunpowder arts.
The very arrival of that technology provided an occasion to use it. Displeased with the ruling Shogun's hospitality toward encroaching Europeans and Americans, the Emperor (who'd been reduced to a figurehead in the last power struggle) rose up and rallied disgruntled provinces against him. Starting in 1864, I took up arms as the Satsuma province -- an Imperial faction in the south of Japan -- to take part in the 10-year war for control of the country.
Bang Bang, You're Dead
I'd rushed in without bothering to invest in more than a couple of firearm-equipped soldiers, because really, how important could they be?
Fall of the Samurai is to Total War: Shogun 2 as Napoleon: Total War is to Empire: Total War: a stand-alone expansion that adds primitive guns and improved naval combat. Eager to give it a try, I haphazardly led my starting army into the territory of my closest neighbor. My first battle, fought in an open green field as I strolled through a neighboring province territory, was a disaster – or "a SHAMEFUL DISPLAY!" as the advisers are fond of describing it. I'd rushed in without bothering to invest in more than a couple of firearm-equipped soldiers, because really, how important could they be? Owowowowow.That's not to say that guns are overpowered, but they definitely did feel essential. Just like only a foolish general would take a group of all footsoldiers up against a well-balanced army that included archers and cavalry, it's unwise to stick exclusively to old-school forces when going against the new.

Finally, we can put on our own production of the 1812 Overture!
With nothing left to lose, I sent my Foreign Veteran agent (anyone can hire one, but they're a starting bonus for Statsuma) striding confidently up to the enemy army and had him slap the general in the face with a glove, challenging him to single combat. At this stage of the game it's a fairly risky move – my low-level Veteran had a less-than-even chance of winning this duel, but heck, that's still better than Vegas odds. My bet paid off, and the enemy general went down like a sack of potatoes.
Gathering Forces
If I'd had more surviving men, that would've been a great time to press an attack against their province. Instead, humbled, I took my battered forces home to lick their wounds, built a Cadet Academy, and trained more advanced units. Assisted by my Foreign Veteran's training bonus, I spent several in-game months building up a capable army before setting out for revenge. (Turns represent shorter periods of time to reflect that this war "only" lasted a decade.) So it was that I marched on the enemy city in the spring of 1865 with a mixed army of 1,500 footsoldiers, gunners, and cavalry.
It gets really interesting when a battle happens within range of the ships' guns.
The opposing general had a similar, but slightly smaller force behind his walls. I hadn't waited to build cannons before attacking, so ordinarily this would be a costly battle for me. However, I'd brought along my small fleet of two ships, hugging the coast to maximize the bombardment coverage. That's a new mechanic for Total War, and on the strategic map it works much like the Civilization equivalent, shelling troops and cities from afar and dealing damage to units and structures. Where it gets really interesting is what happens when a battle takes place within range of the ships' guns. 
He who controls the sea gets to wreak havoc on the land.
When my troops neared the walls, the AI soldiers bunched up to defend, raining a punishing hail of gunfire down on my men. I signaled my ships to fire, and roughly 10 seconds later a barrage of six or seven explosive cannon shells came down all around the enemy. Most missed, leaving only deep impact craters, but one landed dead-center, sending men flying and giving me the opening I needed to storm the walls. A second volley (my last available in this battle) was even more effective, smashing a concentration of soldiers defending the capture point at the center of the fortress. That cleared the way for my troops and left only a handful of enemies for them to mop up.
Gunplay
So will naval units be important? Yes, I'd say so -- especially since my ships were safely parked off-map the entire time, so there was no way to stop the bombardment once the battle was joined. You'll have to control the seas with a fleet of your own and build coastal defense structures in your cities, or else keep your troops on the move to avoid incoming fire.
There's a ton more to Fall of the Samurai than this -- there are all manner of new tweaks, from the political level on down, and battles can potentially be double the size of Shogun 2's. For example, a subtle but important touch that I appreciated is that as a faction's technology improves, the old faithful Samurai warriors actually get penalties that make them weaker in combat. Creative Assembly explains the deterioration with the argument that the emphasis on training and equipping traditional Japanese warriors wasn't what it used to be. Kind of poignant, really.

