In block interleaving you place codewords in rows and read them in columns, and do just the opposite at the receiver side. You can change the depth of the interleaver as you want. However, if your channel is very slow, like quasistatic, then interleaver will not do any thing, because the inteleaved parts will experience the same fading even with interleaving. So, you will get coding gain only without time diversity as you could have in block fading with interleaver or fast fading both with coding.