Can I substitute Fenugreek seeds for leaves?
Fenugreek seeds are intensely aromatic and go quite nicely in curries, and any dish where such a flavour is required.  In the middle-east they use it quite a lot, often as a condiment.  Highly used in spice rubs, pickles, marinades, anything where a strong spice flavour is required and often in puddings as an aromatic like nutmeg is used in English cooking.

The leaves have a similar flavour, but not quite the same, you could use the seeds in their place adding them in to the spice mix if there is one, but the leaves I prefer personally.  The flavour of methi leaves in thepla or in curry is wonderful in my opinion.