You can create an alternative boot script that loads but does not start some applications.

1. Duplicate releases/0.1.0/start.script to releases/0.1.0/load.script
2. Remove one or more {apply,{application,start_boot,[app_name,permanent]}} lines, as necessary
3. Compile the boot script, e.g. elixir -e ":systools.script2boot('releases/0.1.0/load')"
4. Run the release with the alternative boot script, e.g. RELEASE_BOOT_SCRIPT=load ./bin/app_name start_iex

Or you can start with the built-in boot script start_clean and then manually start specific dependencies you need using Application.ensure_all_started(:some_dependency):

$ RELEASE_BOOT_SCRIPT=start_clean ./bin/app_name start_iex
Erlang/OTP 22 [erts-10.7.1] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Interactive Elixir (1.10.1) - press Ctrl+C to exit (type h() ENTER for help)
iex(app_name@localhost)1> Application.ensure_all_started(:ssl)
{:ok, [:crypto, :asn1, :public_key, :ssl]}
iex(app_name@localhost)2>