1 e4 Usually leads to a more dynamic and tactical game than other first moves.
1...c5 Black opts for the Sicilian, the most dynamic and popular response to 1 e4.
2 d4 White avoids the normal continuation 2 Nf3, after which Black can steer the game into a variation which he knows and White probably does not know so well. Instead, White steers the game into his pet variation, which will likely make Black uncomfortable.
2...cxd4 The best way to refute a gambit is to accept it! There are many popular ways of declining the gambit also.
3 c3 Now it becomes a true gambit.
3...dxc3 Black accepts the challenge. In exchange for his gambited Pawn, White will have play on the d and c files.
4 Nxc3 Sometimes White will offer a 2nd Pawn by not taking immediately, but this is said to be "unnecessary and unsound" on White's part.
4...Nc6 Covers e5 and blocks the c file.
5 Nf3 This and Bc4 are likely interchangeable.
5...d6 Will be needed anyway, so why not play it now?
6 Bc4 While White will have an issue about where to develop his other Bishop, the King's Bishop always goes here.
6...e6 Black often blunders here with 6...Nf6?, and after 7 e5! Nxe5?? 8 Nxe5 dxe5 9 Bxf7ch! Black has lost his Queen, while after 7...dxe5 8 QxQ NxQ 9 Nb5 Rb8 10 Nxe5 e6 White stands better.
7 0-0 Normal and good.
7...a6 This prevents Nb5, and thus is an important part of Black's defensive scheme.
8 Qe2 This and White's last are likely interchangeable.
8...Bd7 This, along with Black's next, initiate a line which Joseph Sipman, in a series of articles in Chess Horizons 17 years ago, says is "clearly the best". An alternative for Black is 8...b5 9 Bb3 Ra7!?, initiating the so-called "Chicago Defense". I was introduced to this setup years ago when Steve Carter played it against me in club games. Based on Steve's success with it, I played it myself as Black in a number of ICC games, but found out that White has resources against it. Shipman gives 10 Rd1 Be7 11 Be3 Rd7 12 Nd4! (the refutation) Nxd4 13 Bxd4 Bf6! 14 e5 dxe5 15 Bxe5 Bxe5 16 Qxe5 Nf6 17 Rxd7 Bxd7 18 Qd6!; or 10 Be3! Rd7 11 Nd4!, with the idea that White has kept his King Rook home, where it supports the push f2-f4-f5 with a good attack.
9 Rd1 White proceeds in standard fashion.
9...Be7 Shipman says this defense "poses a real threat to the soundness of the Smith-Morra gambit". By delaying ...Nf6, Black gives White the problem of where to move his Queen Bishop.
10 Be3! Waiting moves here do not help, because Black also has the useful waiting move ...Rc8. 10 Bf4 is also playable, though Black comes out OK if he knows what he's doing. After 10 Bf4
Shipman gives 10...e5! 11 Be3 Nf6 and now: 1) 12 Bg5 Bg4!; 2) Rac1 Ng4!; 3) h3 Rc8!; or 4) Nd5! NxN 13 BxN 0-0 14 Rac1 Rac8
10...Nf6! Not 10...Rc8 11 Na4 b5 12 Bb6 Rc7 and either 13 Bb3 or 13 Bxb5ch are good for White.
11 Bg5! Shipman says this keeps the initiative.
11...Ng4! Gives back the Pawn, but White must trade Queens in the process. The continuation now given by Shipman is 12 Bxe7 Qxe7 13 Qd2! Nge5! 14 Be2! Nxf3 15 Bxf3 Ne5 16 Be2 Bc6! 17 Qxd6 Qxd6 18 Rxd6 Ke7 19 Rd2 g5! 20 Rad1=.