Having interviewed with Alvarez and Marsal in the past, I think I can help you with some specifics here. The rounds are usually as below:
1. 1 telephonic round - more of a fit / guesstimate round
2. 3-4 long cases: they usually provide a written case wherein you have to study the case and prepare your analysis. Could have changed over time
3. Partner rounds: 1-2 partner rounds depending on the level you have applied for. They will mostly check you FIT and ideation part.
With regard to preparation, the following should guide you in the preparation:
1. Principles are the same: The basic principles of structuring and problem-solving remain the same. Solve the case using the methods that you have learnt till now. You might want to brush-up your financial concepts as their cases usually involve some bit of valuation or go/no-go decision (I got one such case along with profit maximization clubbed together).
2. Presentation and packaging: This is where it gets very different from the traditional cases. You have one shot at explaining your thought process. Moreover, since you have time, you will be expected to be rigorous and structured. A&M does a lot of due diligence projects for its Clients. Hence, you could differentiate yourself by packaging it as a "commercial due diligence" exercise (definitely read up on what is a commercial due diligence at A&M). You should be crisp, structured along a thought-process (basic profit structures or a due-diligence structure - whichever suits you) and decisive in your recommendations. Prepare a clear brief note with your structure and recommendations.
3. Questions: Be ready to defend your logic with confidence (not arrogance/defensive). They might grill you on your assumptions so have them ready at hand.
All being said, the written cases are usually very simple with some quantitative (Finance based) and mostly qualitative (typical MBB case-study) aspects. Read the case carefully at least twice before forming any structure in your mind. This will ensure that you don't assume something which is not explicitly said in the case brief.
Let me know if you need any other help with A&M preparation. All the best!