[valarauko]

Explicitly? No. At no point does a member sworn in as an MP have to then swear to affirm their party affiliation. In practice, however, the Anti-Defection Law deems anybody voting against the will of the party to have vacated their membership of the party, which can then be pursued to strip them of membership of the house.

[MichaelZuo]

Is it normal for MPs to speak out against the party line even while voting it?

[valarauko]

Not at all. It's highly unusual for a politician to speak against the party line on a bill, if at all. The only time we ever see fracturing, if any, is around the elections, where dissatisfied members may break away or switch parties. It's not uncommon for people to switch allegiances if they were refused an election ticket as the party candidate, or for higher level politicians to form break away factions if refused the Chief Ministerial position, for example. The Anti-Defection Law allows breakaway factions only if at least 1/3rd leave en masse.