Break a big object up into smaller dependencies that are defined per context. This avoids redefining the big object for each context.

spec.rb

describe Foo do
  let(:big_params) do
    {
      foo: foo,
      name: "name",
      ...
    }
  end

  let(:foo) { nil }

  context "case a" do
    let(:foo) { :a }

    it "test 1" do
      big_params[:foo] # => :a
    end

    it "test 2" do
      big_params[:foo] # => :a
    end
  end

  context "case b" do
    let(:foo) { :b }

    it "test 1" do
      big_params[:foo] # => :b
    end

    it "test 2" do
      big_params[:foo] # => :b
    end
  end
end